Living on the Edge - Wet Use Joist Spans in the IRC R507

[jar546]

Talk about cutting it close; check this out. In the IRC Table R507.6, a 2x8 nominal Southern Yellow Pine floor joist has a maximum span of 11' 10" @ 16" o.c., which is cutting it close, to say the least. There may be a safety factor built in, but certainly not a practicality factor, especially when you look at how close the deflection is to failing.

What are your thoughts on this? Did you think the tables were more conservative than they are?

 

[Yankee Chronicler]

You know what they say: "Close only counts in horseshoes and hand grenades."

 

[wwhitney]

What are your thoughts on this? Did you think the tables were more conservative than they are?

This is the expected outcome when the span is deflection controlled. The safety factors are built into the allowable stresses and the design modulus.

Cheers, Wayne

 

[bill1952]

Codes should not be a guide for good design, but merely a minimum. Meeting code earns a letter grade of D.

 

[jar546]

This is the expected outcome when the span is deflection controlled. The safety factors are built into the allowable stresses and the design modulus.

Cheers, Wayne

I understand & agree. This is the difference between minimum safety and good design for the longevity of the structure.

 

[Paul Sweet]

Wet use would be for decks, which seldom support plaster ceilings. IBC Table 1604.3 allows L/240 deflection for non-plaster ceilings, and L/180 deflection for no ceiling.

 

[jar546]

Wet use would be for decks, which seldom support plaster ceilings. IBC Table 1604.3 allows L/240 deflection for non-plaster ceilings, and L/180 deflection for no ceiling.

I thought we were discussing floor joists. Yes in the IBC as you describe for rafter/ceiling joists, but not in the IRC.

 

[Paul Sweet]

Why would you be analyzing interior floor joists as wet use?

 

[wwhitney]

I understand & agree. This is the difference between minimum safety and good design for the longevity of the structure.

Unless you're running herds of elephants around on these joists, I don't think designing for L/360 deflection is going to decrease the longevity of the structure.

Cheers, Wayne

 

[steveray]

That is the weirdest beam report I've ever seen if it gives you the remainder that you passed by? Using 0.7% instead of 99.3%?